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You could download the electromagnetic transient software PSCAD from the Manitoba Research website. The free student edition will give you enough nodes to model the transformer, feeder back to the PCC, and network infeed. You'll need to know the inrush curve for the transformer. The help pages and examples are fairly detailed.
Regards,
Francis Shillitoe

There's a 2009 CIRED paper "A simplified method for estimating voltage dips due to transformer inrush" that describes the inrush model developed for the IPSA system study program but which also has a 'rule of thumb' calculation that might be good enough for your 300kVA transformer. The problem will be not so much modelling the voltage dip as getting any real data on the transformer. Most manufacturers of distribution transformers seem to have little clue about inrush currents.